Use of a laser to fusion-splice optical components of substantially different cross-sectional areas

ABSTRACT

A method is provided for fusion-splicing with a laser beam at least two optical components to a different optical component, the different optical component (e.g., an optical element such as a lens) having a surface that has a comparatively larger cross-sectional area than a surface of the other optical components (e.g., at least two optical fibers). The method comprises: (a) aligning the optical components along an axis; (b) turning on a directional laser heat source to form the laser beam; (c) directing the laser beam to be collinear with those optical components having a smaller cross-sectional area; (d) ensuring that the laser beam strikes the surface of the optical component having the larger cross-sectional area at normal or near normal incidence so that absorption of the laser beam is much more efficient on the surface; (e) adjusting the power level of the laser beam to reach a temperature equal to or higher than the softening temperature of the surface of the optical component having the larger cross-sectional area to form a softening region thereon, thereby achieving the fusion-splicing; and (f) turning off the laser.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of application Ser.No. 09/118,033, filed Jul. 17, 1998, now U.S. Pat. No. 6,033,515.

TECHNICAL FIELD

The present invention relates generally to optoelectronics involvingoptical components and, more particularly, to coupling opticalcomponents together of significantly different cross-sectional areas,such as coupling optical fibers to optical elements such as lenses,filters, gratings, prisms, and the like.

BACKGROUND ART

Splicing of one optical fiber to another or of one optical fiber to anoptical waveguide is known. Such splicing can be done by a variety oftechniques, including fusion-splicing, which involves localized meltingin the region of the splice.

The following references disclose fusion-splicing of fiber to fiber orfiber to silica-waveguide: (1) R. Rivoallan et al, “Monomode fibrefusion-splicing with CO₂ laser”, Electronics Letters, Vol. 19, No. 2,pp.54-55, 1983; (2) R. Rivoallan et al, “Fusion-splicing of fluorideglass optical fibre with CO₂ laser”, Electronics Letters, Vol. 24,No.12, pp.756-757, 1988; (3) N. Shimizu et al, “Fusion-splicing betweenoptical circuits and optical fibres”, Electronics Letters, Vol. 19, No.3, pp.96-97, 1983; (4) T. Shiota et al, “Improved optical couplingbetween silica-based waveguides and optical fibers”, OFC'94 TechnicalDigest, pp.282-283; and (5) H. Uetsuka et al, “Unique opticalbidirectional module using a guided-wave multiplexer/demultiplexer”,OFC'93 Technical Digest, p. 248-249. In both cases (fiber-fiber orfiber-waveguide), the masses to fuse are very small and of similar size.The fusion does not require careful thermal balance between the twocomponents involved and can be done with a laser beam impinging from theside.

U.S. Pat. No. 4,737,006 entitled “Optical Fiber Termination IncludingPure Silica Lens And Method Of Making Same”, issued to K. J. Warbrick onApr. 12, 1988, discloses fusion-splicing an undoped (pure) silica rod toa single mode fiber to fabricate a collimator, employing an electricarc. However, this is an extremely complicated method and has limitedapplications.

The present practice in the art often requires the attachment of opticalfibers to other optical elements such as lenses, filters, gratings,prisms, and other components which have a much larger cross-sectionalarea than the optical fibers. The most often utilized processes forattaching optical fibers to the larger optical elements include (1)bonding the fiber faces directly to the optical element with adhesivesor (2) engineering a complex mechanical housing which provides stablepositioning of air-spaced fibers and optical elements throughout largechanges in environmental conditions.

The use of adhesives in the optical path of such devices is undesirabledue to the chance of degradation of the adhesive over time. On the otherhand, spacing the fibers a fixed distance away from the optical elementsby utilizing complex mechanical housings requires the use ofanti-reflection coatings at all air-glass interfaces in order tominimize losses of optical energy through the device. The presence ofair-glass interfaces also provides a source of back-reflected light intothe optical fibers. This back-reflected light is a source of noise inmany communication networks, and effectively limits transmissionbandwidth of such communication networks.

In previous art, it has been shown that positioning an angle cleavedfiber or polished fiber in proximity to the angle polished face of acollimating lens results in excellent collimation and excellentperformance characteristics. However, these existing technologies forassembling collimators require very labor intensive active alignmenttechniques. The alignment techniques include manipulating the positionof the fiber relative to the lens in three linear axes and threerotational axes during final assembly. If a collimator can be built thateffectively makes the fiber and the lens a single piece, then alignmentcan be reduced to two linear and two rotational axes during the fusionprocess and there is no need for alignment during final assembly,thereby reducing costs dramatically.

A key performance parameter to be minimized in collimator assemblies isback reflection of light down the fiber. By butt-coupling orfusion-splicing a fiber to a lens of the same refractive index, there isno apparent interface to cause back reflection. The beam is then allowedto diverge in the lens and does not see an index break surface until itexits the lens. By then, the beam is so diffused that the amount oflight that can return to the fiber core is extremely small.

Many advances can be made in the optoelectronics and telecommunicationsmarkets if one is able to fusion-splice a single mode optical fiberdirectly to a collimating lens, a filter, a grating, a prism, awavelength division multiplexer (WDM) device, or any other opticalcomponent of comparatively larger cross-sectional area. More generally,these advances can be made if one is able to fuse optical components ofsubstantially different cross-sectional areas.

Thus, a need remains for a method of fusion-splicing optical componentsof significantly different cross-sectional areas.

DISCLOSURE OF INVENTION

In accordance with the present invention, such a method is provided forfusion-splicing optical components with significantly differentcross-sectional areas using a laser. By “significantly different” ismeant a difference of at least two times.

The method of the present invention for fusion-splicing with a laserbeam at least two optical components to another optical component, thetwo (or more) optical components having a surface that has acomparatively smaller cross-sectional area than a surface of the otheroptical component, comprises:

(a) aligning the optical components along an axis;

(b) turning on a directional laser heat source to form the laser beam;

(c) directing the laser beam to be collinear with the optical componentshaving a smaller cross-sectional area;

(d) ensuring that the laser beam strikes the surface of the opticalcomponent having the larger cross-sectional area at normal or nearnormal incidence so that absorption of the laser beam is much moreefficient on the surface;

(e) adjusting the power level of the laser beam to reach a temperatureequal to or higher than the softening temperature of the surface of theoptical component having the larger cross-sectional area to form asoftening region thereon, which then softens, thereby achieving thefusion-splicing; and

(f) turning off the laser heat source.

The method of the invention is particularly useful for fusion-splicingtwo or more optical fibers to an optical element, such as a lens, havinga much larger cross-sectional area. In the case of the presentinvention, the difference in cross-sectional areas between the opticalfibers and the optical element is at least about two times, andtypically at least about ten times, although the present invention isnot so limited.

Seamlessly fusing the optical fibers to the optical elements, as definedherein, negates the need for both adhesives and complicated housings.Additionally, such fusing eliminates the source of back-reflected light,and requires no additional antireflective coatings between opticalfibers and optical elements. The present invention represents asubstantial improvement to optoelectronic assembly, and allows suchdevices to be manufactured at significantly lower costs than currentlyachievable.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand accompanying drawings, in which like reference designationsrepresent like features throughout the FIGURES.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referred to in this description should be understood as notbeing drawn to scale except if specifically noted.

FIG. 1 is a side elevational view, showing schematically the apparatusemployed in the practice of the present invention; and

FIG. 2 is a view of an annular laser beam as it appears on the surfaceof a mirror through which the optical fibers are passed.

BEST MODES FOR CARRYING OUT THE INVENTION

Reference is now made in detail to a specific embodiment of the presentinvention, which illustrates the best mode presently contemplated by theinventors for practicing the invention. Alternative embodiments are alsobriefly described as applicable.

Localized heat has been effectively used in a variety of glassprocessing operations including surface polishing, fiber drawing, andfusion-splicing. The heat source used is frequently a simple resistanceheater or a controlled arc. All of the aforementioned processes can alsobe performed using a laser as a heat source.

Prior to the present invention, however, a method for splicing opticalcomponents of substantially different cross-sectional areas had not beendeveloped, to the knowledge of the inventors. The present invention isdirected to a method to form seamlessly fused monolithic pieces.

To fuse two or more optical components of a first cross-sectional areato an optical component of a substantially larger cross-sectional area,in one embodiment, the larger surface is first pre-heated by the laser.The pre-heat temperature is just sufficient to polish and melt thesurface of the larger component at the location one desires to fuse thesmaller component. Depending upon the size, it may be a heating of theentire surface or only a localized heating. The second surfaces are thenbrought into contact with the preheated surface and, once the thermalexchange is established (by conduction of heat), all components areheated simultaneously. If all surfaces are large (large with respect tothe localized heating zone), then all may need preheating. Once thesurfaces are in contact at the appropriate elevated temperatures, fusionoccurs. The fusion temperature is just enough above the softeningtemperature to ensure a good flow of thermal energy between the twocomponents.

In a second embodiment, the fusion occurs starting with contact of allof the optical components and the components are never separated duringthe fusion-splicing.

In a third embodiment, all of the optical components are brought intocontact, then pulled back after alignment, and then fusion-spliced as inthe first embodiment.

Qualification of the interface is accomplished by measuring the backreflection of light through the system as well as mechanical testing.

There are no practical limitations in using this technique with respectto size mismatch, or the absence of a mismatch, or in cross-sectionalgeometry.

Any multiple pieces of optical elements, whether comprising an inorganicglass or an organic polymer, can be fused using the method of thepresent invention. The most common application will be fusion of singlemode fibers to optoelectronic or telecommunications devices.Fusion-splicing in accordance with the teachings herein virtuallyeliminates back-reflection and the associated losses. It is also verycost-effective, with a splice requiring a few seconds or less and theprocess can be fully automated. Splicing eliminates the need for activealignment in many instances. Splicing also ablates contaminants andprecludes the need for foreign materials, such as adhesives and otherorganic materials, in the optical path.

Optical inorganic glasses, such as silicas, borosilicates, borates,phosphates, aluminates, chalcogenides and chalco-halides, halides, etc.,and optical organic polymers, such as acrylates, methacrylates, vinylacetates, acrylonitriles, styrenes, etc., may be beneficially employedin the practice of the present invention, although the invention is notlimited to the specific classes of materials listed.

Because the heating is quick and localized, components can beanti-reflection-coated on surfaces other than the surface to be fusedprior to fusion. The process of the present invention also minimizes thenumber of coated surfaces. Typical assembly techniques leave a minimumnumber of surfaces to be coated: the face of each optical fiber beingspliced and both the input and output faces of the lens. However, theprocess of the present invention leaves as few as one surface becauseseveral surfaces (each optical fiber face and the lens input face) arecombined into a monolithic fused piece. Every surface, even when coated,contributes losses to the system because there is no perfectantireflection coating. Thus, reducing the number of surfaces to becoated reduces losses to the system.

Pointing accuracy and beam quality can be monitored prior to fusion andlocked in due to fusion. Because the part count and the labor intensityof the process is minimized, costs are very low.

Elimination of angled surface index breaks reduces polarization effectssuch as polarization-dependent losses (PDL) and polarization modedispersion (PMD) in fabricated components. Current methods employoptical surfaces which are angled relative to the optical axis in orderto control back reflection, thereby inducing PDL and PMD above thoseinherent in the materials.

Another distinct advantage of the present invention is the thermalstability of the system. Because the parts are seamlessly fused into amonolithic piece, there is no dependence on the housing for maintainingsub-micron spacing tolerances as there is with other prior artapproaches in optoelectronic and telecommunications devices.

The present invention makes possible a very high quality and low costproduct for the optoelectronics/telecommunications industry. Withoutthis technology, one would be forced to use the prior art techniquesknown in the telecommunications industry, which are very costly, cannotperform as well, and/or use undesirable materials in the optical path.

The novel method of the present invention for splicing two or more smallcross-sectional area optical components (e.g., optical fibers) to alarger cross-sectional area optical component (e.g., optical element)comprises:

1. aligning the optical fibers and the optical element along an axis;

2. turning on a directional laser heat source (such as an infraredlaser) to form a laser beam;

3. directing the laser beam to be collinear with the fibers (this way,most of the laser light is not absorbed by the small fibers but isreflected off surface because the reflection coefficient is very high atgrazing incidence);

4. ensuring that the laser beam strikes the larger cross-sectional areaoptical element at normal or near normal incidence so that absorption ofthe laser is much more efficient on the larger surface;

5. adjusting the laser power level to reach a temperature equal to orhigher than the softening temperature on the surface of the element toachieve fusion-splicing (and simultaneously achieve polishing andcontamination ablation); and

6. turning off the laser.

In the first embodiment, the two components (e.g., optical fibers andoptical element) are aligned but separated by a space (typically a fewmillimeters), the laser beam is turned on to form the softening region,and the surface of the optical components having the smallercross-sectional area is brought in contact with the softening region ofthe optical component having the larger cross-sectional area, thecontact resulting in heat transfer to the surface of the opticalcomponents having the smaller cross-sectional area, which then softens,thereby achieving the fusion-splicing.

In the second embodiment, the two components (e.g., optical fibers andoptical element) are first brought into contact and the laser beam isthen turned on to form the softening region where the two components arein contact to achieve the fusion-splicing.

In the third embodiment, the two components (e.g., optical fibers andoptical element) are aligned, then brought into contact, then separatedby a space (typically a few millimeters), the laser beam is turned on toform the softening region, and the surface of the optical componentshaving the smaller cross-sectional area is brought in contact with thesoftening region of the optical component having the largercross-sectional area, the contact resulting in heat transfer to thesurface of the optical components having the smaller cross-sectionalarea, which then softens, thereby achieving the fusion-splicing.

For fusion-splicing typical inorganic glasses, such as silica, a CO₂laser, which operates in the range 9 to 11 μm, is preferred, sincesilica-based glasses have very large absorption coefficient. Otheroptical materials typically have a large absorption in the infrared, andaccordingly, lasers operating in another region of the IR spectrum maybe used with such other optical materials.

The laser beam is collinear and grazes the optical fibers. This can beaccomplished in many ways. For example, a 45-degree mirror with acentral hole is used to redirect the laser beam colinerar with the axesof the fibers (the fibers pass through the hole, parallel to eachother). Other methods that direct the laser beam along the axis of thefibers may also be employed; such methods are well-known to thoseskilled in this art. The laser beam itself can be (but not necessarily)annular in shape. This last requirement is accomplished by differenttechniques: scanning system, special optical components (axicon), TEM₀₁laser mode, central obstruction, diffractive optical element, etc. Thesame effect could be accomplished by using two or more laser beams, allcollinear with the optical fibers.

The optical components being fusion-spliced preferably have similarthermal and/or mechanical properties. However, this is not a necessaryrequirement, since dissimilar optical components can be fusion-splicedemploying the teachings of the present invention. In such cases, thepossibility of strain due to the process may cause the splice to breakif the conditions are not right, and thus must be taken into account.However, such a consideration is well within the experience of theperson skilled in this art, and no undue experimentation is required.

FIG. 1 depicts the laser beam 10 impinging on the mirror 12, which has ahole 12 a therethrough. Two optical fibers 14 a, 14 b pass through thehole 12 a in the mirror 12 and are fusion-spliced to the optical element16. FIG. 1 depicts the optical fibers 14 a, 14 b just prior tofusion-splicing to the lens 16. FIG. 2 depicts an annular laser beam 10a in cross-section, along with the two fibers 14 a, 14 b. The opticalelement 16 may be a lens, filter, grating, prism, WDM device, or othersuch optical component to which it is desired to secure the opticalfibers 14 a, 14 b. While two optical fibers 14 a, 14 b are depicted inthe drawings, it will be appreciated by those skilled in this art thatmore than two optical fibers may be fusion-spliced to an opticalcomponent 16, based on the teachings herein.

The technology disclosed herein can be applied to conventional fibercollimators, expanded beam collimators, WDM products, and any otherdevice that has a glass or polymer attachment site. One is no longerlimited to fusing components that only have substantially similardiameters.

Industrial Applicability

The method of the invention is expected to find use in fusion-splicingat least two optical components having a relatively smallercross-sectional area to an optical component having a relatively largercross-sectional area, such as splicing two or more optical fibers to anoptical lens.

Thus, there has been disclosed a method for fusion-splicing opticalcomponents together of dissimilar cross-sectional areas, such assplicing two or more optical fibers to an optical element. It will bereadily apparent to those skilled in this art that various changes andmodifications of an obvious nature may be made, and all such changes andmodifications are considered to fall within the scope of the presentinvention, as defined by the appended claims.

What is claimed is:
 1. A method for fusion-splicing at least two opticalcomponents to a different optical component with a laser beam, saiddifferent optical component having a surface that has a comparativelylarger cross-sectional area than a surface of the other opticalcomponents, comprising: (a) aligning said optical components along anaxis; (b) turning on a directional laser heat source to form said laserbeam; (c) directing said laser beam to be collinear with those opticalcomponents having a smaller cross-sectional area; (d) ensuring that saidlaser beam strikes said surface of said optical component having saidlarger cross-sectional area at normal or near normal incidence so thatabsorption of said laser beam is much more efficient on said surface;(e) adjusting the power level of said laser beam to reach a temperatureequal to or higher than the softening temperature of said surface ofsaid optical component having said larger cross-sectional area to form asoftening region thereon, thereby achieving said fusion-splicing; and(f) turning off said laser heat source.
 2. The method of claim 1 whereinsaid larger cross-sectional area is at least two times larger than saidsmaller cross-sectional area.
 3. The method of claim 2 wherein saidlarger cross-sectional area is at least ten times larger than saidsmaller cross-sectional area.
 4. The method of claim 1 wherein saidoptical components comprise silica-based glasses.
 5. The method of claim4 wherein said laser operates in a wavelength region of about 9 to 11μm.
 6. The method of claim 5 wherein said laser is a CO₂ laser.
 7. Themethod of claim 1 wherein said optical component having said largercross-sectional area is an optical element.
 8. The method of claim 1wherein said optical components having said smaller cross-sectional areaare optical fibers.
 9. The method of claim 8 wherein said directing ofsaid laser beam to be collinear with said optical components having saidsmaller cross-sectional area is achieved by providing a mirror having ahole therethrough, through which said optical fibers pass.
 10. Themethod of claim 9 wherein said mirror is inclined at 45-degrees withrespect to said optical fibers.
 11. The method of claim 1 wherein saidcomponents are aligned but separated by a space, said laser beam isturned on to form said softening region, and said surface of saidoptical components having said smaller cross-sectional area is broughtin contact with said softening region of said optical component havingsaid larger cross-sectional area, said contact resulting in heattransfer to said surface of said optical components having said smallercross-sectional area, which then softens, thereby achieving saidfusion-splicing.
 12. The method of claim 1 wherein said components arefirst brought into contact and said laser beam is then turned on to formsaid softening region where said components are in contact to achievesaid fusion-splicing.
 13. The method of claim 1 wherein said componentsare aligned, then brought into contact, then separated by a space, saidlaser beam is turned on to form said softening region, and said surfaceof said optical components having said smaller cross-sectional area isbrought in contact with said softening region of said optical componenthaving said larger cross-sectional area, said contact resulting in heattransfer to said surface of said optical components having said smallercross-sectional area, which then softens, thereby achieving saidfusion-splicing.
 14. A method for fusion-splicing at least two opticalfibers to an optical element with a laser beam, said optical elementhaving a surface that has a comparatively larger cross-sectional areathan a surface of said optical fibers, comprising: (a) aligning saidoptical fibers and said optical element along one axis; (b) turning on adirectional laser heat source to form said laser beam; (c) directingsaid laser beam to be collinear with said optical fibers; (d) ensuringthat said laser beam strikes said surface of said optical element atnormal or near normal incidence so that absorption of said laser beam ismuch more efficient on said surface; (e) adjusting the power level ofsaid laser beam to reach a temperature equal to or higher than thesoftening temperature of said surface of said optical element to form asoftening region thereon, thereby achieving said fusion-splicing; and(f) turning off said laser heat source.
 15. The method of claim 14wherein said cross-sectional area of said optical element is at leasttwo times larger than that of said optical fibers.
 16. The method ofclaim 15 wherein said cross-sectional area of said optical element is atleast ten times larger than that of said optical fibers.
 17. The methodof claim 14 wherein said optical element and said optical fibers eachcomprise silica-based glasses.
 18. The method of claim 17 wherein saidlaser operates in a wavelength region of about 9 to 11 μm.
 19. Themethod of claim 18 wherein said laser is a CO₂ laser.
 20. The method ofclaim 14 wherein said directing of said laser beam to be colinear withsaid optical fibers is achieved by providing a mirror having a holetherethrough, through which said optical fibers pass.
 21. The method ofclaim 20 wherein said mirror is inclined at 45-degrees with respect tosaid optical fibers.
 22. The method of claim 14 wherein both saidoptical element and said optical fibers have similar thermal andmechanical properties.
 23. The method of claim 14 wherein said opticalelement is selected from the group consisting of lenses, filters,gratings, prisms, and wavelength division multiplexer devices.
 24. Themethod of claim 14 wherein said optical element and said optical fibersare aligned but separated by a space, said laser beam is turned on toform said softening region, and said surface of said optical fibers isbrought in contact with said softening region of said optical element,said contact resulting in heat transfer to said surface of said opticalfibers, which then softens, thereby achieving said fusion-splicing. 25.The method of claim 14 wherein said optical element and said opticalfibers are first brought into contact and said laser beam is then turnedon to form said softening region where said optical fibers contact saidoptical element to achieve said fusion-splicing.
 26. The method of claim14 wherein said optical element and said optical fibers are aligned,then brought into contact, then separated by a space, said laser beam isthen turned on to form said softening region, and said surface of saidoptical fibers is brought in contact with said softening region of saidoptical element, said contact resulting in heat transfer to said surfaceof said optical fibers, which then softens, thereby achieving saidfusion-splicing.